The present invention relates to a radiation-sensitive semiconductor device having a semiconductor body wherein a first semiconductor region of a first conductivity type has one side at the surface of the semiconductor body and adjoins within the semiconductor body at a certain distance from and substantially parallel to the surface with a second semiconductor region of a second conductivity type opposite to the first type and forming a first pn junction therewith. The invention further relates to a reading or writing unit comprising such a radiation sensitive device.
Such radiation-sensitive semiconductor devices area necessary component of an optical recording system, such as the so-called Compact Disk (CD), laser vision (LV) and Compact Disk Video (CDV), for which information can be read optically, or the Digital Optical Recording (DOR), wherein information is written and read. In such systems, electromagnetic radiation is applied to the information carrier and information is obtained from the reflected radiation by detecting it by means of a radiation-sensitive semiconductor device of the type referred to in the foregoing. Especially, use when is to be made of a number (for example four) of such semiconductor regions and associated pn junctions, these devices are arranged adjacent to each other.
Such a radiation-sensitive semiconductor device is known from the English abstract of Japanese Kokai 61-154063 (date of publication July 12, 1986), which is published in Pat. Abstracts of Japan, Vol. 10, Nov. 29, 1986, No. 356 (E-459), p. 26 under No. E-45926. A semiconductor body is described therein having a first semiconductor region of the n-conductivity type, which is disposed in an epitaxial layer of the n-conductivity type adjoining the surface and which adjoins within the semiconductor body at a certain distance from and parallel to the surface an epitaxial layer of the p-conductivity type and forms a pn junction therewith.
A disadvantage of the known radiation-sensitive semiconductor device is that, when several of such pn junctions are arranged beside each other in the semiconductor body--for example to detect different parts of a radiation beam--, the separation region of such pn junctions--viewed laterally--causes a large dip in the radiation sensitivity. Two pn junctions are separated laterally in the known semiconductor device by a region of the p-conductivity type sunken into the epitaxial layer of the n-conductivity type. When the distance of the pn junction from the surface is, for example, 10 .mu.m--a distance necessary for radiation having a large depth of penetration--, the width of this insensitive region is also about 10 .mu.m. This is very objectionable especially for beams of radiation commonly used in practice which have a diameter of a few tens of microns. The same disadvantage also occurs when the separation is constituted by an etched groove. Another disadvantage of the known semiconductor device is due to the required presence of a separation circuit comprising capacitors having a capacitance of about 40 pF between the amplifiers connected to the pn junctions and constructed as a split-band amplifier. The amplifiers comprise on the one hand four D.C.-coupled amplifiers each amplifying a current through a pn junction, with which the so-called focus error signal is formed, and on the other hand one A.C.-coupled amplifier for amplifying the sum of the currents through the pn junctions, with which the so-called data signal is formed. When these capacitors are integrated, comparatively large parasitic capacitances are obtained with respect to the substrate, which is undesirable for the amplification of the data signal. A further disadvantage of the known radiation-sensitive semiconductor device consists is the limited possibility of integration of the most suitable amplifiers for the focus error signal. For this signal, the most suitable amplifiers are so-called Class B amplifiers, which have a small offset. When the substrate is of the p-conductivity type, the first semiconductor region forms part of an epitaxial layer of the n-conductivity type and the Class B amplifier must comprise pnp transistors. Pnp transistors having accurately defined properties cannot readily be manufactured in an n-type epitaxial layer because they must be formed laterally. For an n-type substrate and a p-type epitaxial layer, the same holds for the npn transistors then required.